In recent years, a flexible device in which a functional element such as a semiconductor element, a display element, or a light-emitting element is provided over a substrate having flexibility (hereinafter also referred to as a flexible substrate) has been developed. Typical examples of the flexible device include, as well as a lighting device and an image display device, a variety of semiconductor circuits including a semiconductor element such as a transistor.
As a method for manufacturing a device including a flexible substrate, a technique has been developed in which a functional element such as a thin film transistor or an organic electroluminescence (hereinafter also referred to as EL) element is formed over a formation substrate (e.g., a glass substrate or a quartz substrate), and then the functional element is transferred to a flexible substrate. This technique needs a step of peeling a layer to be separated including the functional element from the formation substrate.
In this specification, a step of peeling a layer to be separated from a formation substrate by physical force or a step of peeling the formation substrate from the layer to be separated by physical force is referred to as a substrate peeling process. Furthermore, a method for peeling a layer to be separated from a formation substrate by physical force or a method for peeling the formation substrate from the layer to be separated by physical force is referred to as a substrate peeling method. These are the step and the method for peeling a necessary structure with less damage, which are different from those for peeling unnecessary objects, for example, for peeling a resist or the like by photolithography or the like.
For example, Patent Document 1 discloses the following peeling technique using laser ablation: a separation layer formed of amorphous silicon or the like is formed over a substrate, a layer to be peeled which is formed of a thin film element is formed over the separation layer, and the layer to be peeled is bonded to a transfer body with a bonding layer. The separation layer is ablated by laser light irradiation, so that peeling is generated in the separation layer.
Peeling and transfer techniques described in Patent Documents 2 and 3 are proposed. Patent Document 2 discloses a peeling technique in which a silicon oxide film serving as a peeling layer is removed by wet etching. In addition, in Patent Document 3, a peeling technique by which a silicon film that is to be used as a peeling layer is removed by dry etching is disclosed.
Patent Document 4 discloses a technique in which peeling and transfer are conducted by physical force such as human hands. Patent Document 4 discloses a technique in which a metal layer (Ti, Al, Ta, W, Mo, Cu, Cr, Nd, Fe, Ni, Co, Ru, Rh, Pd, Os, or Ir) is formed over a substrate and an oxide layer is stacked thereover. In this technique, when the oxide layer is formed, a metal oxide layer of the metal layer is formed at an interface between the metal layer and the oxide layer, and peeling is carried out in a later step by utilizing this metal oxide layer.